Printed circuit boards (PCB) are manufactured using a large number of complex steps and processes. Basically, the PCB is made by laminating thin sublayers together. Copper clad laminates are normally used to make the etched circuitry layers, and are bonded together with prepreg or bonding sheets. The resulting single strong panel may optionally have internal etched copper layers centrally embedded in the resin. The panel is then drilled, and the drilled holes are metallized to provide the electrical connections between the inner and the outer layers. The outer two layers are then etched to make the circuitry patterns. The resulting structure is the well-known plated through hole, shown in FIG. 1. Note that the interior layers are connected to the metallized hole, which also further connects them to the exterior two layers.
Artisans have sought to use ever-decreasing hole diameters in order to reduce the real estate on the board surface. However, it is costly to produce small diameter plated through holes with guaranteed electrical continuity, because plating metal in these tiny holes is not a reproducible process. The quality of the connection between the hole plating and the internal layers can only be checked by destructive microsectioning. One alternative to small-hole PCB technology is multilayer ceramic. The substrates for these modules have many (up to 23) layers, and each layer begins as a part of a continuous cast sheet of ceramic material, cut into pieces 175 mm square, then punched with holes so that electrical connections could later be made between layers. Conductive paste is then extruded onto the green sheets through metal masks, forming a wiring pattern unique to a given layer. Stacks of these sheets, with the required configurations of conducting line and insulating layers, layer-to-layer connections, and reference and power planes, were laminated together and trimmed to form individual modules, which were then fired in a furnace to harden the ceramic. The resulting small diameter via contains a conductive media that forms the side to side connection.
Clearly, a need exists for a better, more efficient way to create small diameter plated through holes.